Two-step build-up of a thermoreversible polymer network: From early local to late collective dynamics

TL;DR

This study investigates the formation of thermoreversible gel networks, revealing two distinct stages of physical gelation characterized by local nucleation and long-range cooperative aging, influenced by covalent crosslinks.

Contribution

It introduces a detailed analysis of gelation stages using hybrid gelatin gels with controlled covalent crosslinks, highlighting the transition from local to collective dynamics.

Findings

Identified two gelation stages separated by a crossover modulus.

Rearrangements govern aging, even with covalent crosslinks.

Late aging is logarithmic and slowed but not halted by covalent bonds.

Abstract

We probe the mechanisms at work in the build-up of thermoreversible gel networks, with the help of hybrid gelatin gels containing a controlled density of irreversible, covalent crosslinks (CL), which we quench below the physical gelation temperature. The detailed analysis of the dependence on covalent crosslink density of both the shear modulus and optical activity evolutions with time after quench enables us to identify two stages of the physical gelation process, separated by a temperature dependent crossover modulus: (i) an early nucleation regime during which rearrangements of the triple-helix CL play a negligible role, (ii) a late, logarithmic aging one, which is preserved, though slowed down, in the presence of irreversible CL. We show that aging is fully controlled by rearrangements and discuss the implication of our results in terms of the switch from an early, local dynamics to a late, cooperative long-range one.